What does the 'D' stand for in 'D-class live power amplifiers'? Some people would say it stands for 'digital', but that's actually a misconception! The truth is it doesn't actually stand for anything; it was just the next letter in the alphabet after the earlier-developed C-class amplifiers. Technically, a D-class power amp is simply an amp which uses switching transistors instead of linear gain ones - but without going into the nuts and bolts of the equipment, what it boils down to is that these amplifiers are lighter in weight, run cooler and can even be physically smaller than other classes. Which means it's no surprise they're so popular for use in live sound systems!
If you have a favorite brand, that would be a good starting point to help you track down the best D-class live power amp for you. Behringer fans, for example, could get started at the entry level with the Behringer iNUKE NU1000 Power Amp, or turn things up to '11' with the Behringer iNUKE NU6000DSP. Peavey ups the ante a little steeper with the Peavey IPR2 DSP 5000 Power Amp with DSP while also keeping themselves grounded in affordable performers like the Peavey IPR2 3000 DSP Power Amp. And of course, let's not forget QSC: their amplifiers, including the best-selling QSC PLD4.3 Multi-Channel System Processing Amplifier and the QSC PLD4.5 Multi-Channel System Processing Amplifier, go a long way to set the standard for this category of power amp.
So what are you looking for in your live power amp? If the answer is 'something that's lightweight, efficient, easy to set up and that won't bake itself in the rack,' then D-class live power amplifiers are probably the way to go. In fact, the more amps your system takes, the more the advantages pile up for D-class amps. And with a whole slew of great units here to choose from, it's safe to say you can find a good fit for your system no matter how you prefer to lay things out.